Creatures and lands you control untap at the same time as the active player’s permanents. You can’t choose to not untap them at that time.

9/15/2013

Effects that state a creature or land you control doesn’t untap during your untap step won’t apply during another player’s untap step.

9/15/2013

Controlling more than one Prophet of Kruphix doesn’t allow you to untap any creature or land more than once during a single untap step.

9/15/2013

If you’re casting a creature spell with bestow, whether or not it has flash will depend on if you’re casting it as a creature or as an Aura. Prophet of Kruphix won’t give flash to a spell with bestow you’re casting as an Aura.

9/15/2013

The last ability applies to creature cards in any zone, provided something is allowing you to cast them. For example, if you control a Zombie, you could cast Gravecrawler (a creature with “You may cast Gravecrawler from your graveyard as long as you control a Zombie”) from your graveyard as though it had flash.